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FAN2535S26X 数据表(PDF) 6 Page - Fairchild Semiconductor |
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FAN2535S26X 数据表(HTML) 6 Page - Fairchild Semiconductor |
6 / 11 page FAN2534/FAN2535 PRODUCT SPECIFICATION 6 REV. 1.0.2 8/26/03 Control Functions Enable Pin Connecting 2.0V or greater to the Enable pin will enable the output, while 0.4V or less will disable it while reducing the quiescent current consumption to less than 1µA. If this shutdown function is not needed, the pin can simply be connected permanently to the VIN pin. Allowing this pin to float will cause erratic operation. Error Flag (FAN2535-XX Only) Fault conditions such as input voltage dropout (low VIN), overheating, or overloading (excessive output current), will set an error flag: The ERR pin which is an open-drain output, will go LOW when VOUT is less than 95% or the specified output voltage. When the voltage at VOUT is greater than 95% of the specified output voltage, the ERR pin is HIGH. A logic pullup resistor of 100K Ohm is recommended at this output. The pin can be left discon- nected if unused. Thermal Protection The FAN2534/FAN2535 is designed to supply high peak output currents for brief periods, however sustained exces- sive output load at high input - output voltage difference will cause the device temperature to increase and exceed maximum ratings due to power dissipation. During output overload conditions, when the die temperature exceeds the shutdown limit temperature of 155°C, an onboard thermal protection will disable the output until the temperature drops approximately 15°C below the limit, at which point the output is re-enabled. During a thermal shutdown situation the user may assert the power-down function at the Enable pin, reducing power consumption to a minimum. Thermal Characteristics The FAN2534/FAN2535 is designed to supply 150mA at the specified output voltage with an operating die (junction) temperature of up to 125°C. Once the power dissipation and thermal resistance is known, the maximum junction tempera- ture of the device can be calculated. While the power dissipa- tion is calculated from known electrical parameters, the actual thermal resistance depends on the thermal characteris- tics of the SOT23-5 surface-mount package and the surrounding PC Board copper to which it is mounted. The power dissipation is equal to the product of the input-to- output voltage differential and the output current plus the ground current multiplied by the input voltage, or: The ground pin current IGND can be found in the charts provided in the Electrical Characteristics section. The relationship describing the thermal behavior of the package is: where TJ(max) is the maximum allowable junction tempera- ture of the die, which is 125°C, and TA is the ambient operat- ing temperature. θJA is dependent on the surrounding PC board layout and can be empirically obtained. While the θJC (junction-to-case) of the SOT23-5 package is specified at 130°C /W, the θJA of the minimum PWB footprint will be at least 235°C /W. This can be improved upon by providing a heat sink of surrounding copper ground on the PWB. Depending on the size of the copper area, the resulting θJA can range from approximately 180°C /W for one square inch to nearly 130°C /W for 4 square inches. The addition of backside copper with through-holes, stiffeners, and other enhancements can also aid in reducing thermal resistance. The heat contributed by the dissipation of other devices located nearby must be included in the design consider- ations. Once the limiting parameters in these two relation- ships have been determined, the design can be modified to ensure that the device remains within specified operating conditions. If overload conditions are not considered, it is possible for the device to enter a thermal cycling loop, in which the circuit enters a shutdown condition, cools, re- enables, and then again overheats and shuts down repeatedly due to an unmanaged fault condition. Adjustable Version The FAN2534 adjustable version includes an input pin ADJ which allows the user to select an output voltage ranging from 2.5V to near VIN, using an external resistor divider. The voltage VADJ presented to the ADJ pin is fed to the onboard error amplifier which adjusts the output voltage until VADJ is equal to the onboard bandgap reference voltage of 1.00V(typ). The equation is: Since the bandgap reference voltage is trimmed, 1% initial accuracy can be achieved. The total value of the resistor chain should not exceed 250K Ohm total to keep the error amplifier biased during no-load conditions. Programming output voltages very near VIN need to allow for the magni- tude and variation of the dropout voltage VDO over load, supply, and temperature variations. Note that the low-leak- age FET input to the CMOS Error Amplifier induces no bias current error to the calculation. P D V IN V OUT – ()I OUT V INIGND + = P D max () T J max () TA – θ JA ------------------------------- = V OUT 1.00V 1 R upper R lower ---------------- + × = |
类似零件编号 - FAN2535S26X |
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类似说明 - FAN2535S26X |
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